Forged scroll parts and production process thereof

ABSTRACT

An object of the present invention is to provide an aluminum-alloy-made forged scroll part in which harmful primary Si crystals are not formed and variation in wrap height between the scrolls is low. The present invention also provides a process for producing the forged scroll part. An alloy material including Si: 8.0-12.5%; Cu: 1.0-5.0%; Mg: 0.2-1.3%; and if necessary, Ni: 2.0% or less and/or one or more species selected from among Sr, Ca, Na, and Sb: total 0.5% or less, is cast through continuous casting into a round bar material having a diameter of 130 mmφ or less, and subsequently, the material is subjected to upsetting and hot forging with back pressure to produce a forged scroll part containing substantially no Si particles having a size of 15 μm or more, the mean size of Si particles in the forged part being 3 μm or less.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofProvisional Application 60/230,807 filed Sep. 7, 2000 pursuant to 35U.S.C. §111(b).

FIELD OF THE INVENTION

[0002] The present invention relates to an aluminum alloy-made forgedpart for an orbiting scroll and/or a fixed scroll, which is assembledinto a scroll compressor employed mainly in an air conditioner; and to aprocess for producing the forged part.

BACKGROUND OF THE INVENTION

[0003] In recent years, scroll compressors have become of interest asair conditioner compressors. One reason is that such a scroll compressorcontains a small number of parts and is driven silently. The scrollcompressor includes a fixed scroll having a spiral wrap portion 11, asshown in FIG. 1, and an orbiting scroll having a spiral wrap portionwhose shape is similar to that of the portion 11. The spiral wrapportion of the orbiting scroll is driven for orbital movement so thatthese spiral wrap portions face each other.

[0004] In many cases a fixed or orbiting scroll (hereinafter simplyreferred to as a “scroll”), which serves as a main part of a scrollcompressor, is produced from aluminum alloy in order to reduce theweight of the compressor. The scroll is produced by, for example,casting or forging. In order to provide a scroll with strength andreliability, forging is advantageously carried out for producing thescroll. Since the scroll has a complicated shape, it must be producedthrough hot forging.

[0005]FIG. 2 shows a conventional production process for an aluminumalloy scroll part by forging.

[0006] Usually, a round bar material obtained through extrusion isemployed as a stock material for forging. Firstly, an aluminum alloyprepared by mixing alloy components and melting is cast throughcontinuous casting into a billet (BL) for extrusion having a largediameter of 200 mmφ or more (“φ” used herein represents “diameter”.).After the inside of the BL is homogenized through heat treatment, the BLis cut into pieces such that they have identical volumes to provideround bars, each having predetermined length and diameter, and eachpiece is subjected to extrusion to form a round bar.

[0007] Usually, the diameter of the extruded round bar is almost equalto the outer diameter of a forged part. The round bar is cut intopieces, and the pieces are employed as a stock material for forging. Asdescribed below, if necessary, the cut piece may be previously shaped byforging or machining into a piece having a shape similar to that of thescroll part in order to facilitate production of a scroll part, beforeforging of the stock material to employ the shaped piece as a stockmaterial for forging.

[0008] The stock material is forged into a scroll part usually throughhot forging. In order to provide the forged part with strength, theforged part is usually subjected to solution (quenching) and aging heattreatment after forging.

[0009] Thereafter, if necessary, a portion of the surface of the part issubjected to machining in order to enhance precision in the size of theforged part.

[0010]FIG. 4 is a schematic cross-sectional view showing a conventionalforging process for a scroll. A workpiece 4 placed in a die 2 is presseddownward with a punch 1 to form the wrap portion 11. Usually, thedistance that the punch 1 moves is determined to be consistent in orderto make the thickness of a flange portion 12 of the scroll consistent.

[0011] In order to precisely forge a workpiece into a scroll wrap,Japanese Patent Application Laid-Open (kokai) Nos. 54-159712, 59-61542,and 62-89545 disclose a process for forging an aluminum alloy-madescroll, in which the workpiece is subjected to forging or machining inadvance to provide the piece with a previous shape, and then theworkpiece is forged into the scroll wrap. The reason why such a processis carried out is as follows. The wrap portion 11 has a spiral shape,the height of the portion is large, and the wrap portion is connected tothe flange portion 12. Therefore, when a workpiece is forged into ascroll as shown in FIG. 4, forming a wrap portion having a uniformheight is difficult, and thus a workpiece having an intermediate shapeis formed in advance. When the process is carried out, the producedscroll is provided with a shape with some degree of precision. However,the process requires designing an intermediate shape which matches thefinal shape of the scroll, and preparation of a forging die employed forintermediate processing. Consequently, the process includes complicatedsteps and involves high costs, presenting difficulty in practice.

[0012] Japanese Patent Application Laid-Open (kokai) Nos. 60-102243 and06-23474, among other publications, disclose a back pressure forgingprocess in which a workpiece prepared only by cutting a round bar isemployed without subjecting the workpiece to pre-processing beforeforging, and, during forging of the workpiece, a load is applied to theend portion of a scroll wrap 11 in a direction opposite a forgingdirection in order to control material flow to realize a uniform flowinto a wrap-shaped mold and to reduce variation in the height of thescroll wrap 11. According to the process, by using a workpiece preparedonly by cutting a round bar, a scroll in which there is a reduction invariation in the height of a wrap portion 11 can be produced at low costand high productivity.

[0013] The process will be described in more detail. In the backpressure forging process for a scroll, as shown in cross-sectional viewsof FIGS. 5 and 6, a workpiece 4 is pressed with a punch 1, and theworkpiece is forged into a die space for wrap formation 2 a of a die 2to form a wrap. During forging, a back pressure lower than a punchpressure is applied through knock pins 7 and knockouts 6 to the end ofthe wrap in a direction opposite that of forging thereby making theheight L2 of the wrap uniform, as shown in a cross sectional view of aforged part (FIG. 7).

[0014] The back pressure forging process regulates, to some extent, theeffect for making the height of a spiral wrap of a forged scroll partuniform.

[0015] However, although variation in the height of a wrap of a scrollis regulated to some extent through the back pressure forging process,wrap height varies between individual scrolls unless the thickness ofindividual cut materials; i.e., the weight of individual workpieces, isstrictly controlled. Therefore, a margin for machining of the end of awrap must be controlled in every forged part during a post-processingstep. Alternatively, in consideration of different wrap heights amongscroll products, slightly-large-sized scrolls must be forged to providescrolls with a large margin for machining during a post-processing step,resulting in low yield.

[0016] In the forging process, when a workpiece is forged into a scroll,the thickness (L1) of a flange portion is controlled by a stroke of thepunch 1, and the remaining portion of the workpiece is forged into awrap portion. Therefore, the difference in the volume of the workpiecebefore forging is reflected in the difference in the height (L2) of thewrap portion.

[0017] Conventionally, in order to smoothly carry out forging of aworkpiece without production loss, the workpiece is prepared by cuttinga round bar material having a diameter nearly equal to the maximum outerdiameter of a forged scroll (i.e., the outer diameter of a flangeportion). Therefore, variation in the thickness of the cut material isreflected in variation in volume of the workpiece; i.e., variation inthe height of a wrap portion of the scroll.

[0018] The area of a horizontal cross section of a wrap portion is about⅓ to ⅕ that of a horizontal cross section of a workpiece. Accordingly,the variation in the cut length of the workpiece is multiplied by afactor of 3 to 5 in height of the wrap portion. Therefore, since amargin of the end of the wrap for machining in a post-processing stepcannot be reduced, the amount of time for machining of scrolls cannot bereduced, and material-based yield cannot be enhanced.

[0019] In consideration of conditions under which scrolls are used, analuminum alloy material containing a large amount of silicon is employedfor producing a scroll to enhance strength and wear resistance of thescroll. The material is hard, and thus a blade for cutting the materialis easily worn. Therefore, compared with a conventionally-used alloy,variation in the thickness of the aluminum alloy material increasesduring cutting, greatly affecting variation in wrap height betweenindividual forged scrolls.

[0020] As described above, an aluminum alloy material is employed forproducing a scroll in order to reduce the weight of the scroll. Inconsideration of the balance between strength, wear resistance, andprocessability, Al-Si alloy materials have mainly been developed among avariety of aluminum alloy materials. When characteristics of thematerial are regulated to impart wear resistance to the material, fineSi particles are uniformly dispersed in an aluminum base. Development ofalloy materials other than Al-Si alloy materials has encountereddifficulty at present. Therefore, such an alloy material is not employedin practice, and basically modifications of Al-Si alloy materials hasbeen carried out.

[0021] In such an Al-Si alloy material, crystallization of Si particlesis necessary for enhancing wear resistance of the material. However,crystallization of coarse primary Si crystals having a size of tens ofμm or more causes wear of a blade during machining, resulting in aproduct to having a rough machined surface. In addition, when suchcoarse primary Si crystals segregate at a portion of a scroll subjectedto high stress, fatigue breakage initiates at the portion where thescroll is employed, greatly impairing reliability of the scroll.Furthermore, as described above, when such an Al-Si alloy material iscut, wear of a blade is accelerated. Thus, variation in the thickness ofthe material increases during cutting. Therefore, such an Al-Si alloymaterial preferably has a structure in which crystallization of coarseprimary Si crystals is suppressed and fine eutectic Si particles havinga size of several μm are uniformly dispersed.

[0022] As described above, in a conventional production process, such analuminum alloy material is formed through cutting into an extrusionround bar material. In order to form the round bar material, the alloymaterial is usually cast through continuous casting into a billet (BL)having a relatively large diameter (200 mmφ or more). Therefore, thebillet is solidified slowly during casting, and thus crystallization ofcoarse primary Si crystals having a size of 100 μm or more tends tooccur, and control of the distribution of Si particles in the billet isdifficult. Furthermore, as described above, variation in the thicknessof the billet may occur during cutting. In addition, primary Si crystalsremain in a forged scroll product as a large, hard impurity, and thecrystals may cause problems in machining of the forged scroll andreduction in strength thereof.

SUMMARY OF THE INVENTION

[0023] The present invention provides a forged scroll part employed in ascroll compressor and a production process for the scroll part, which isproduced from an aluminum alloy material which enables reduction invariation of wrap height within a forged part and between forged parts,reduction in a margin for machining in post-processing, and suppressionof occurrence of coarse primary Si crystals that would cause wear of ablade during machining and reduction in strength of the forged scrollpart.

[0024] In order to solve the aforementioned problems, the presentinvention provides:

[0025] (1) a forged scroll part produced from an aluminum alloy materialcomprising:

[0026] Si: 8.0-12.5 mass %;

[0027] Cu: 1.0-5.0 mass %; and

[0028] Mg: 0.2-1.3 mass %,

[0029] wherein the scroll part contains substantially no Si particleshaving a size of 15 μm or more, and the mean Si particle size is 3 μm orless.

[0030] If necessary, the forged part may further comprise:

[0031] Ni: 2.0 mass % or less; and/or

[0032] one or more species selected from among Sr, Ca, Na, and Sb: total0.5 mass % or less.

[0033] The strength of the forged part is enhanced through solution heattreatment, quenching, and aging, and, if necessary, the forged part issubjected to machining and is imparted with characteristics satisfactoryfor a practically-used scroll part.

[0034] The present invention also provides:

[0035] (2) a process for producing the forged part, which comprises astep for casting an aluminum alloy material into a round bar having adiameter of 130 mmφ or less through continuous casting of an aluminumalloy material comprising Si: 8.0-12.5 mass %, Cu: 1.0-5.0 mass %, andMg: 0.2-1.3 mass %; a step for cutting the aluminum alloy round bar intoa stock material for forging; a step for subjecting the stock materialto upset at an upsetting ratio of 20-70% to form a pre-shaped product(hereinafter “a workpiece”); and a forging step for applying pressureonto the workpiece with a punch at a temperature of 300-450° C. to forma scroll wrap in a direction of punch pressing, wherein the forging stepincludes a single step in which a forged scroll part is press-formedwhile back pressure is applied to the end of the wrap of scroll part ina direction opposite that of a punch pressure.

[0036] The present invention also provides:

[0037] (3) a process for producing the forged part, which comprises astep for casting an aluminum alloy material into a rod having a diameterof 85 mmφ or less through continuous casting of an aluminum alloymaterial comprising Si: 8.0-12.5 mass %, Cu: 1.0-5.0 mass %, and Mg:0.2-1.3 mass %; a step for cutting the aluminum alloy round bar into astock material for forging; a step for subjecting the stock material toupset at an upsetting ratio of 20-70% to form a workpiece; and a forgingstep for applying pressure onto the workpiece by use of a punch at atemperature of 300-450° C. to form a scroll wrap in a direction of punchpressing, wherein the forging step includes a single step in which aforged scroll part is press-formed while back pressure is applied to theend of the wrap of scroll part in a direction opposite that of a punchpressure.

[0038] In the aforementioned production process, the alloy material mayfurther comprise Ni: 2.0 mass % or less, and/or one or more speciesselected from among Sr, Ca, Na, and Sb: total 0.5 mass % or less.Preferably, after completion of forging, the forged part is subjected tohomogenization heat treatment at 480-520° C. for 30 minutes to fourhours, and/or to surface peeling.

[0039] Preferably, a work lubrication process in which the workpiece iscoated with graphite film in advance is carried out in combination witha die lubrication process in which a graphite-containing oily lubricantis applied to a die as a lubrication process during forging.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a schematic representation of a forged scroll part.

[0041]FIG. 2 is a flow chart of a process in which a conventionalmaterial for extrusion is employed as a stock material for forging.

[0042]FIG. 3 is a flow chart of the process of the present invention.

[0043]FIG. 4 is a schematic cross-sectional view showing a conventionalforging process for a scroll.

[0044]FIG. 5 is a cross-sectional view of a material, a punch, and a diebefore the forging step of the present invention.

[0045]FIG. 6 is a cross-sectional view of a material, a punch, and a dieduring the forging step of the present invention.

[0046]FIG. 7 is a schematic representation showing a cross-sectionalview of a forged scroll part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The present invention will be described in more detail.

[0048] The aluminum alloy scroll is usually produced from anSi-containing aluminum alloy in order to impart wear resistance to thescroll. Crystallization of fine particles of added Si enhances wearresistance of the scroll against another scroll.

[0049] When the content of Si contained in the alloy is about 11 mass %or less, fine eutectic Si particles having a size of several μmdispersedly crystallize in an Al base in proportion to the content ofSi, and the Si particles enhance wear resistance of the alloy scroll.Therefore, the content of Si is preferably high. When the content of Sicontained in the alloy is less than 8.0 mass %, a sliding part, such asa scroll formed from the alloy, exhibits unsatisfactory wear resistance.

[0050] In contrast, when the content of Si contained in the alloy is inexcess of 12.5 mass %, crystallization of primary Si crystals occurs.The primary Si crystals tend to become large to have a size as large astens of μm. The large Si crystals cause wear of a blade during cutting,and cause loss of the edge of a cutting tool during machining inpost-processing, resulting in a problem in finishing. In addition, whenthe crystals segregate at a portion in the vicinity of the outer surfaceof a forged part, which is susceptible to stress concentration, breakageof the forged part initiates at the portion, resulting in lowering ofmechanical strength. Therefore, the upper limit of Si content is 12.5mass %.

[0051] When Cu is added to the aluminum alloy in an amount of several %,strength of the Al base is enhanced through post heat treatment.Addition of Cu also contributes to enhancement of wear resistance of thealloy. However, when the content of Cu contained in the alloy is lessthan 1.0 mass %, Cu does not contribute to enhancement of strength ofthe alloy. When the content of Cu is in excess of 5.0 mass %, Cu doesnot contribute to enhancement of strength of the alloy commensurate withthe content of Cu. Therefore, the content of Cu is preferably 1.0-5.0mass %.

[0052] Mg combines with Si, precipitating in the form of Mg₂Si in thealloy after heat treatment, and this precipitation contributes tohardening of the alloy. Mg also forms MgSiCu through precipitation afterheat treatment, and the compound contributes to hardening of the alloy.Such Mg compounds enhance the strength of the alloy. When the content ofMg is less than 0.2 mass %, Mg fails to exert such a effect. When thecontent of Mg is in excess of 1.3 mass %, the effect of Mg does notincrease commensurate with the content of Mg. In addition, an oxidegenerates and invades the alloy during casting, resulting in defects ofthe alloy. Therefore, the content of Mg is preferably 0.2-1.3 mass %.

[0053] If necessary, the alloy of the present invention may furthercontain Ni in an amount of 2.0 mass % or less. Addition of a smallamount of Ni exerts an effect of enhancing heat resistance of the alloy.When the content of Ni is 0.1 mass % or less, Ni fails to exert theabove effect. When the content of Ni is in excess of 2.0 mass %, largecrystals generate, resulting in lowering of toughness of the alloy.Therefore, the content of Ni is preferably 0.1-2.0 mass %.

[0054] In the alloy of the present invention, eutectic Si particlescontribute to enhancement of wear resistance of the alloy. In order touniformly disperse the Si particles in the alloy and to suppressgeneration of coarse primary Si crystals, the alloy may contain one ormore species selected from among Sr, Ca, Na, and Sb in a total amount of0.5 mass % or less. Preferably, Sb is contained in an amount of 0.05-0.5mass %, and Sr is contained in an amount of 0.005-0.05 mass %. Sr isparticularly preferable, since addition of a trace amount of Sr exertsthe above effect, and weight loss of the alloy during melting is small.

[0055] A characteristic feature of the present invention resides in thealloy composition or in the process including a melting step of thealloy and a forging step, particularly in the steps of the process. Withreference to FIG. 2, there will be described a conventional productionprocess for a forged part by use of a material for extrusion as a stockmaterial for forging. In the conventional process, an aluminum alloyround bar for extrusion is cut into pieces and the piece is employed asa stock material for forging, since a flange portion of a scroll has around shape and has an outer diameter of about 80-130 mmφ. Firstly, thealuminum alloy is melted, and cast through continuous casting intobillets for extrusion. Each billet is subjected to homogenization heattreatment, and cut into pieces having a length of tens of cm, and eachpiece is extruded through an extrusion machine into a round bar having adiameter nearly equal to that of a scroll. Subsequently, the round baris cut in a direction perpendicular to the longitudinal direction of thebar to obtain a stock material for forging. The material is heated, alubricant is applied to the material, and then the material is subjectedto hot forging.

[0056] When the alloy is cast into a usual billet for extrusion, thebillet usually has a diameter as large as 200 mmφ or more. Therefore,since the billet is cooled slowly and solidified gradually,crystallization of coarse primary Si crystals having a size of about 100μm easily occurs when the content of Si in the alloy is in excess of10%. Consequently, the crystals may remain in the extruded round bar ofsmall diameter. The primary Si crystals tend to segregate at the centerportion of the billet, at which the cooling rate of the billet isparticularly low. When the content of Si is nearly equal to 12%, theprimary Si crystals generate randomly in the entirety of the crosssection of the billet.

[0057]FIG. 3 shows an example of the process of the present invention.In the present invention, in order to avoid generation of coarse primarySi crystals, an aluminum alloy is cast through continuous casting into around bar material having a diameter of 130 mmφ or less. In contrast toa conventional billet for extrusion, a round bar material havingdiameter of 130 mmφ or less, which is obtained through continuouscasting, is cooled very rapidly and thus solidified rapidly. Therefore,eutectic Si particles become fine in the round bar material, and evenwhen the content of Si is in excess of 10 mass %, coarse primary Sicrystals, which generate in the conventional billet, do not generate inthe round bar material. Particularly, in the case in which theaforementioned modification elements, such as Sr, Ca, Na, and Sb, areadded to the alloy in an amount up to the Si content of 12.5 mass %,generation of primary Si crystals is substantially not observed in theround bar material; i.e., the aforementioned problem is avoided. Theround bar material substantially comprises no Si particles when Siparticles having a size of 15 μm or more are substantially not observedin the following process.

[0058] According to the process using the alloy composition, eutectic Siparticles having a size of 15 μm or more are substantially not observedand the particle size is usually about 10 μm at most. The mean particlesize is 3 μm or less. As used herein, the phrase “substantially notobserved” refers to “the percentage of non-observation in a field ofview under a microscope is 99% or more.” In the present invention, theround bar material substantially comprises no Si particles having a sizeof 15 μm or more since Si particles are not substantially observed inthe foregoing process.

[0059] The particle size of Si may be directly determined from aphotomicrograph of the round bar material. Preferably, the particle sizeis obtained through image processing by use of a microscope imageanalyzer (e.g., Luzex), since a correct value is obtained through thistechnique. As used herein, the term “particle size” refers to thediameter of a circle having the same area as that of the particle.

[0060] The diameter of a cast round bar material is preferably small,since the material having a small diameter is solidified rapidly. Whenthe diameter of the material is small, eutectic Si particles in thematerial easily become fine, and generation of primary Si crystals isgreatly suppressed. Therefore, a round bar material having a diameter of85 mmφ or less is more preferable as a cast material from the viewpointthat such a material exhibits excellent upsetting effect as describedbelow.

[0061] The material of the present invention may be cast to have adiameter that matches the outer diameter of a scroll product, and cutinto a stock material for forging. A characteristic feature of thepresent invention is that the cast material has a diameter smaller thanthat of the outer diameter of a scroll product; the cast material is cutto have a length corresponding to the weight of a forged scroll part;and the cut material is subjected to upsetting to attain a desireddiameter. The diameter of the material after upsetting is determined tomatch the outer diameter of a flange portion of the scroll product.Through cutting of the continuously cast bar material having a smalldiameter and upsetting of the cut material, the material exhibitsimproved ductility and fatigue characteristics, due to uniformdispersion of Si particles.

[0062] Upsetting of the cut round bar material may be carried outthrough free-forging; i.e., through application, under two punches, ofpressure onto the material in a vertical direction to sufficientlyenlarge the diameter of the material. However, upsetting of the materialis preferably carried out through die-forging, in which the outerdiameter of the material is determined by a die, in order to enhanceprecision in the diameter and the thickness of the material and to carryout scroll forging, which is the next step, at high productivity.

[0063] During upsetting, the upsetting ratio of the material isappropriately 20-70%.

[0064] The upsetting ratio is obtained by the following formula:

[0065] Upsetting ratio (%)

[0066] =100×(cross-sectional area of material afterprocessing—cross-sectional area of material beforeprocessing)/cross-sectional area of material after processing

[0067] =100×(height of material before processing—height of materialafter processing)/height of material before processing

[0068] Usually, upsetting may be carried out at room temperature whenthe upsetting ratio is low. Preferably, upsetting is carried out afterthe material is heated, since the upsetting ratio can be increased.However, even in the case in which upsetting is carried out at elevatedtemperature, when the upsetting ratio is very high, cracking occurs onthe circumferential surface of the material beyond material ductility.In addition, since the ratio of the height of the material to the outerdiameter thereof becomes high, buckling of the material occurs duringupsetting, and thus a high-quality upset material cannot be obtained.Therefore, in the case of the material of the present invention, theupsetting ratio is appropriately 70% or less, preferably 60% or less.When the upsetting ratio is less than 20%, the material may fail toexhibit improved ductility and fatigue characteristics. In addition,variation in characteristics of the below-described material for forgingis not reduced satisfactorily.

[0069] When upsetting is carried out, the material is usually heated.The material may be heated before upsetting, and then subjected toupsetting. However, in order to improve the surface condition of thematerial during peeling and facing as described below and to enhance theshapability of the material during upsetting, the material is preferablysubjected to homogenization heat treatment before upsetting. Thehomogenization heat treatment is appropriately carried out at 480-520°C. for 30 minutes to four hours. When the temperature is lower than 480°C., the material is not satisfactorily homogenized. When the temperatureis higher than 520° C., eutectic fusion occurs at boundaries betweencrystal particles. The temperature is preferably 495-510° C. When thetreatment time is less than 30 minutes, the material is notsatisfactorily homogenized. When the time is in excess of four hours,eutectic Si particles tend to become large.

[0070] If necessary, the surface of the material may be subjected topeeling and facing in advance. Through peeling and facing, precision inthe diameter of the material is enhanced, and the condition of thecircumferential surface of a workpiece after upsetting is improved.

[0071] The process for casting an aluminum alloy into a round barmaterial having a small diameter, which includes cutting the castmaterial into a stock material for forging; and subjecting the stockmaterial to upsetting to form a workpiece, has the following threeadvantages.

[0072] A first advantage is that generation of primary Si crystals issuppressed and eutectic Si particles become fine in the cast material,since the material is cooled rapidly as described above. When the castmaterial is subjected to plastic working to some extent, the materialexhibits improved ductility and fatigue characteristics.

[0073] A second advantage will be described in relation to the followingreason. Variation in the length of the cut cast round bar material leadsto variation in the volume (weight) of the stock material for forging,which results in variation in the height of a wrap portion of a forgedscroll part. The cast round bar material is usually cut by use of around sawing machine. When the cast round bar has a small diameter, thematerial is accurately fed into the sawing machine to determine thelength of the material, and thus variation in the length (thickness) ofthe cut material tends to be low. In addition, when the diameter of thecast round bar material is small, the area of the cross section of thematerial is small. Therefore, even if variation in the length(thickness) of the material occurs, the variation in the volume (weight)of the material is low compared with that in a material having a largerdiameter. More specifically, when the diameter of the cast round barmaterial is small, variation in the volume (weight) of the stockmaterial for forging becomes low, resulting in low variation in theheight of a wrap portion of a forged scroll part.

[0074] A third advantage is enhancement of material-based yield. Whenthe round bar material is cut into the stock material for forging havinga predetermined length, unwanted pieces are obtained from both ends ofthe bar material, and powdery chips are generated. The amount of loss ofthe material attributed to the powdery chips is determined by thethickness of a cutting blade and the diameter of the round bar material.Specifically, when different stock materials having the same volume arecut from round bar materials having different diameters, the amount ofpowdery chips which are formed when a stock material is cut from a roundbar material having a large diameter is larger than that of powderychips which are formed when a stock material is cut from a round barmaterial having a small diameter. Therefore, when a stock material forforging is cut from a round bar material having a small diameter, lossof the material is reduced. As a result, the stock material for forgingcan be obtained at high yield, which leads to an economical benefit.

[0075] In view of the foregoing, when the upsetting ratio is low duringupsetting of the stock material for forging, the aforementionedadvantages are obtained to an unsatisfactory degree. Therefore, theupsetting ratio is 20% or more, preferably 40% or more.

[0076] A pre-shaped material which has undergone the aforementionedupsetting, i.e., a workpiece, is subjected to hot forging. The diameterof the workpiece is determined to match the outer diameter of a flangeportion of a scroll part.

[0077] An aluminum alloy material is subjected to hot forging at300-450° C., preferably at 350-450° C. When the hot forging temperatureis very low, the material fails to be formed into a predetermined shapeor cracking occurs in the material, whereas when the temperature is veryhigh, swelling or buckling of the material may occur.

[0078] When a workpiece is subjected to hot forging, in order to preventseizing of a workpiece into a forging die, a lubricant is usuallyapplied to the workpiece and the die. In general, when an aluminum alloymaterial is subjected to hot forging, a liquid lubricant containing amixture of graphite and water or mineral oil is widely employed.Usually, when a workpiece is forged into a product having a simpleshape, satisfactory lubrication and release effects are obtained throughmere spraying of a lubricant directly onto a forging die. However, inthe case in which a workpiece is forged into a product having acomplicated shape, when lubrication is not carried out thoroughly, alubricant becomes short, and thus the workpiece is forged into apoorly-shaped product, or the workpiece is seized into a die and cannotbe forged into the product. In order to solve such problems, a workpieceis immersed into a liquid lubricant in advance to coat the piece with alubrication film. When a workpiece is forged into a scroll having acomplicated shape, the workpiece is forged in a die having a wrap-shapeddeep groove to form a wrap portion having a large height. Therefore,since a lubricant fails to cover the entirety of the wrap-contouredinner walls of the die when only spraying is carried out, shaping andrelease of the workpiece is not satisfactorily carried out; i.e.,forging of the workpiece is difficult. In order to solve such a problem,preliminary immersion of the workpiece into the lubricant is carried outin combination with spraying of the lubricant onto the die. As a result,improved lubrication and release effects are obtained, and forging athigh productivity is realized.

[0079] In order to coat a workpiece having lubrication film thereon, asolution prepared by mixing a solvent with a graphite lubricant isapplied to the workpiece. In order to increase productivity, a lubricantprepared by diluting the solution with a rapid-drying solvent is appliedor sprayed to the workpiece.

[0080] In a most economical process, a lubricant is prepared by mixingand dispersing graphite powder into water serving as a solvent, aworkpiece is heated and then immersed into the lubricant, and theresultant workpiece is dried. In this case, the workpiece must be heatedat a temperature at which water serving as a solvent is evaporated ordried within a very short time. When the heating temperature of theworkpiece is lower than the boiling point of water, the lubricant failsto dry and remains on the workpiece after immersion of the workpiece;i.e., the lubricant is not rapidly dried. Therefore, the heatingtemperature of the workpiece must be 100° C. or higher. In considerationof productivity, the heating temperature is preferably 130° C. orhigher. The upper limit of the heating temperature may be a temperatureat which deterioration of the workpiece, such as melting, does notoccur. Briefly, the heating temperature is 500° C. or lower, preferably450° C. or lower. The workpiece is usually heated in a heating furnace.Alternatively, the residual heat of the workpiece after hot upsettingmay be utilized; i.e., the workpiece is immersed into a lubricantimmediately after upsetting. In this case, the lubricant is baked ontothe workpiece which has undergone upsetting, and the workpiece isremoved from the lubricant and then dried.

[0081] Through this procedure, cutting, heating, upsetting, lubrication,and forging may be carried out successively, resulting in highproductivity.

[0082] Upsetting and forging may be carried out simultaneously in asingle pressing apparatus. In this case, continuous production of ascroll part is possible through carrying out cutting, heating,lubrication, upsetting, and forging successively.

[0083] A workpiece that has undergone upsetting and lubrication isforged into a scroll as follows. If necessary, the workpiece isadditionally heated, and the workpiece is pressed downward with a punch1 into a die space 2 a to form a wrap portion downward in the die space2 a. Before the workpiece is pressed with the punch 1, knockouts 6connected through knock pins 7 to a back pressure apparatus areinserted, in advance, in the die space 2 a for forming a wrap, such thatthe knockouts 6 reach the vicinity of the upper end of the die space 2a. When the workpiece is pressed into the die space 2 a to form a wrap,pressure is applied to the end of the wrap in a direction opposite thepressing direction from the back pressure apparatus through a backpressure plate 3, the knock pins 7, and the knockouts 6 to form the wraphaving a uniform height.

[0084] When the back pressure is not applied to the workpiece duringforging, the amount of the metal that flows into the wrap-formingportions in the die can become nonuniform. Therefore, the object ofapplying the back pressure is to obtain a uniform amount of metal flowinto the wrap-forming portions in the die. The amount of the backpressure can regulate the uniformity of the amount of the metal flowinto the wrap-forming portions in the die. Accordingly, by applying theappropriate back pressure to the workpiece, the amount of metal flowinto the wrap-forming portions in the die can be uniform, and thus theheight of the wrap portions of the product can be uniform. When the backpressure is very high, buckling of the wrap occurs during wrapformation, and a good product is not obtained. Therefore, when a forgedpart such as a scroll, in which the ratio of the area of a horizontalcross-section of a wrap portion to that of a horizontal cross-section ofa flange portion is about ⅓ to ⅕, and the height of the wrap portion is4 to 10 times the thickness of the wrap portion, is formed at theaforementioned heating temperature, the surface pressure applied to theend of the wrap is appropriately 40-120 N/mm², preferably 60-100 N/mm².

[0085] In order to impart strength and wear resistance to the forgedscroll part, the scroll part must be subjected to solution (quenching)and aging treatment. The solution temperature is preferably 490-500° C.After the scroll part is subjected to quenching in water, the scroll issubjected to aging for hardening under appropriate conditions; i.e.,temperature: 160-210° C., time: 1-8 hours. Through this procedure, thescroll part is imparted with a satisfactory hardness of about HRB 70-85.

[0086] If necessary, the heat-treated forged scroll part is furthersubjected to machining to precisely regulate the height and the shape ofthe wrap portion. The thus-produced scroll part can be provided in acompressor or the like.

EXAMPLES

[0087] The present invention will next be described by way of Examples.Unless indicated otherwise herein, all parts, percents, ratios and thelike are by weight.

[0088] Production of a workpiece for forging of the present invention

[0089] Alloy materials A through F shown in Table 1 were employed inExamples 1 through 8, and alloy materials G and H shown in Table 1 wereemployed in Comparative Examples 5 and 6. Each alloy material was castinto a bar having a diameter of 82 mmφ and a length of 5,000 mm throughcontinuous casting at a casting rate of about 300 mm/minute. The bar wassubjected to homogenization heat treatment at 500° C. for one hour, andthen subjected to facing by use of a peeling machine to attain adiameter of 78 mmφ.

[0090] Subsequently, the bar was cut into workpieces having a thicknessof 65 mm by use of a round saw having a thickness of 2.5 mm.

[0091] Each workpiece was heated at about 400° C. in a heating furnace,and the disk-shaped workpiece was pressed with a punch into a die by useof a 630-ton press machine to attain a diameter of 114 mmφ to upset theworkpiece through die-forging. The upsetting ratio is obtained from thefollowing calculation:

[0092] upsetting ratio={1-(78/114)²}×100=53%.

[0093] When the bar was cut into workpieces, chips (45 g per workpiece)were formed.

[0094] Production of a workpiece for forging through a conventionalprocess

[0095] Alloy materials B and C shown in Table 1; i.e., alloy materialsof Examples 4 and 5, were employed in Comparative Examples 3 and 4. Eachalloy material was cast into a billet for extrusion having a diameter of200 mmφ through continuous casting at a casting rate of about 150mm/minute. The billet was subjected to homogenization heat treatment at500° C. for one hour, and then extruded into a stock material having anouter diameter of 114 mmφ, which is equal to that of the above upsetworkpiece. The stock material was cut into workpieces having a thicknessof 30.4 mm, so that that the volume of each workpiece was the same asthat of the above upset workpiece, by use of a round saw having athickness of 2.5 mm.

[0096] When the stock material was cut into workpieces, chips (80 g perwork piece) were formed. The amount of loss of the material was abouttwice the loss of the material in the cases of Examples 1 through 8 inwhich the round bar obtained through continuous casting was cut intoworkpieces.

[0097] Observation of internal metallographical structure of a workpiecefor forging

[0098] Subsequently, in order to observe the internal metallographicalstructure of each of the above-prepared workpieces, or to measure thesize and the weight of the workpiece, 10 upset workpieces or 10 cutworkpieces were collected as samples.

[0099] After the sizes and the weights of these 10 workpieces weremeasured, a 20 mm-square sample was cut out of the center portion ofeach workpiece having a diameter of 114 mmφ, and the internalmicrostructure of the sample was observed.

[0100] Through this observation, the existence of primary Si crystals,the size of the crystals, the number of the crystals, and the size ofeutectic Si particles were measured. The weight of each sample wasmeasured by use of an even balance. The thickness of each sample wasmeasured at two points per sample by using of a micrometer. The resultsare shown in Table 2. The weight and the thickness are shown by a rangeof 10 samples.

[0101] The results reveal that, when a workpiece is subjected toupsetting, coarse primary Si crystals are not formed in the workpiece,variation in the size and the weight of the workpiece is reduced, andproduction yield is improved; i.e., a highly-reliable workpieceexhibiting high precision in size can be produced economically.

[0102] Scroll forging

[0103] Subsequently, the above upset workpiece and the aboveextruded-and-cut workpiece were heated at 200° C. in a heating furnace,and then each workpiece was immersed into a water-containing graphitelubricant for several seconds, then removed therefrom to coat theworkpiece with a lubrication film.

[0104] While the workpiece was heated at 400° C., the workpiece wassubjected to forging at a punch pressure of 450 tons and at a backsurface pressure of 40-120 N/mm² to produce a scroll having a flangediameter of about 115 mmφ, a flange thickness of about 23.0 mm, a wrapheight of 39.6 mm, and a wrap thickness of 5.7 mm. The ratio of the areaof a horizontal cross section of the flange to that of a horizontalcross section of the wrap was about 4.0.

[0105] In Comparative Examples 1 and 2, upset workpieces obtained fromalloy material A were subjected to forging at back pressures of 30 and130 N/mm², respectively.

[0106] Under the aforementioned conditions, 50 workpieces of eachExample or each Comparative Example were successively subjected toforging to produce 50 scroll parts. Difference in the height (themaximum height—the minimum height) of the scroll wrap of each forgedpart was measured to obtain variation in wrap height difference betweenthe 50 forged parts. In addition, the mean height of the wrap of eachforged part (the mean value of the heights of the wrap measured at threepoints 11 a, 11 b, and 11 c shown in FIG. 1, wherein 11 a represents aspiral initiation point; 11 c represents a spiral termination point; and11 b represents a point on a line joining the points 11 a and 11 c, thepoint 11 b being adjacent to the point 11 c) was measured to obtainvariation in mean wrap height between the 50 forged parts. Furthermore,the shape of the wrap of each forged port was observed.

[0107] The results are shown in Table 3. The results reveal that, whenthe back pressure is 30 N/mm², difference in wrap height of one forgedpart is in excess of 1 mm; i.e., the height of the wrap becomesnon-uniform when the back pressure is low. In contrast, when the backpressure is 130 N/mm², buckling of the wrap occurs, and a good forgedpart is not produced.

[0108] The results reveal that variation in mean wrap height betweenforged parts produced from workpieces obtained through the conventionalprocess including extrusion and cutting is 1.0 mm or more. That is, asshown in Table 2, variation in volume between the workpieces causesvariation in wrap height between the forged parts.

[0109] According to the present invention, variation in height of thewrap of one forged part falls within 0.5 mm, and variation in mean wrapheight between forged parts also falls within 0.5 mm. That is, a forgedpart having a good shape can be produced.

[0110] Subsequently, 10 forged parts of each of Examples 4 and 5 andComparative Examples 3 through 6 were heated at 500° C., and thensubjected to quenching in water. Subsequently, the parts were subjectedto aging treatment at 180° C. for six hours. Thereafter, a tensile testpiece was obtained from each forged part, and tensile characteristics ofthe forged part were evaluated. In addition, the side wall of the wrapof each forged part was machined about 0.5 mm by use of an end mill, andthen the machined surface was observed. Furthermore, the workpiece forforging was subjected to heat treatment in a manner similar to that ofthe above procedure, and a fatigue test piece was obtained from theworkpiece. The fatigue test piece was subjected to a test by use of anOno-type rotating bending fatigue test apparatus, and fatiguecharacteristics of the workpiece were evaluated on the basis of breakagestress at 10⁷ cycles. The results are shown in Table 4.

[0111] The results reveal that, when an upset stock material is employedas a workpiece, the fracture elongation of the workpiece was improved,and thus a forged part exhibiting high fatigue strength and having anexcellent machined surface was produced. That is, when formation ofcoarse primary Si crystals was suppressed, the above effects areobtained.

[0112] In order to observe the internal metallographical structure ofthe forged part, a test piece was cut out from the central portion ofthe forged part of each of Examples 1 through 8 after aging treatment,and the test piece was subjected to observation of microstructure.Consequently, primary Si crystals were not observed in each test piece,and change in the size of eutectic Si particles attributed to forgingand heat treatment was not confirmed.

[0113] In Comparative Examples 5 and 6, in which the Si content of thealloy material falls outside the range of the present invention,scratches were formed on the machined surface of a forged part, thescratches were attributed to formation of primary Si crystals, and thestrength of the forged part was lowered. Such a forged part is notsuitable for a scroll. TABLE 1 Alloy material (unit: wt %) subjected totest and production process for forged part Stock material Forging forforging back Chemical analysis value (mass %) Diameter pressure AlloyTest No. Si Cu Mg Ni Sb Sr Others (mmφ) Working N/mm² A Example. 1 10.22.9 0.5 — — — Bal. 82 Upsetting 80 Example. 2 10.2 2.9 0.5 — — — Bal. 82Upsetting 40 Example. 3 10.2 2.9 0.5 — — — Bal. 82 Upsetting 120 Comp.Ex. 1 10.2 2.9 0.5 — — — Bal. 82 Upsetting 30 Comp. Ex. 2 10.2 2.9 0.5 —— — Bal 82 Upsetting 130 B Example. 4 11.5 4.5 0.6 — — — Bal 82Upsetting 80 Comp. Ex. 3 11.5 4.5 0.6 — — — — 200 Extrusion 80 CExample. 5 10.4 2.6 0.3 — — — Bal 82 Upsetting 80 Comp. Ex. 4 10.4 2.60.3 — — — Bal. 200 Extrusion 80 D Example. 6 8.9 2.1 0.4 — 0.22 — Bal 82Upsetting 80 E Example. 7 12.0 1.2 1.1 1.2 0.25 — Bal 82 Upsetting 80 FExample. 8 11.2 4.6 0.7 — — 0.01 Bal 82 Upsetting 80 H Comp. Ex. 5 13.14.8 0.5 — — — Bal. 82 Upsetting 80 G Comp. Ex. 6 7.0 0.3 0.2 — — — Bal.82 Upsetting 80

[0114] TABLE 2 Metallographical observation and size measurement ofworkpiece for forging Internal microstructure Primary Si crystalEutectic Si particle Maximum Mean Maximum Size Note size size sizeDiameter Thickness Weight Alloy Test No. Number (μm) (μm) (μm) (mmφ)(mm) (g) Examples A Ex. 1 None — 2.0 4.8 114.0 30.40-30.49 841-843 B Ex.4 None — 2.1 6.7 114.0 30.35-30.51 845-848 C Ex. 5 None — 2.0 4.4 114.030.38-30.52 840-842 D Ex. 6 None — 1.9 4.4 114.0 30.37-30.50 839-842 EEx. 7 None — 2.1 7.2 114.0 30.42-30.52 841-843 F Ex. 8 None — 2.1 5.3114.0 30.44-30.51 845-847 Comparative B Comp. Ex. 3 5 100 2.5 10.3 114.030.20-30.58 844-850 Examples C Comp. Ex. 4 2  52 3.0 15.5 114.030.33-30.63 840-845 H Comp. Ex. 5 5 110 2.0 8.4 114.0 30.37-30.46845-848 G Comp. Ex. 6 None — 1.8 4.8 114.0 30.41-30.49 840-842

[0115] TABLE 3 Size measurement and observation of forged part in eachtest Difference in wrap height 50 Forged parts Back in one forged Meanwrap pressure part/mm height/mm Alloy Test Work piece N/mm² (Max.-Min.)Minimum Maximum Note Example A Ex. 1 Upset piece 80 0.3 to 0.4 39.4 39.7Ex. 2 Upset piece 40 0.3 to 0.5 39.0 39.4 Ex. 3 Upset piece 120 0.2 to0.4 39.2 39.5 B Ex. 4 Upset piece 80 0.3 to 0.4 39.2 39.6 C Ex. 5 Upsetpiece 80 0.3 to 0.4 39.4 39.7 D Ex. 6 Upset piece 80 0.3 to 0.4 39.239.7 Comparative A Comp. Upset piece 30 1.3 to 2.0 — — Variation inExample Ex. 1 wrap height Comp. Upset piece 130 0.2 to 0.4 39.0 39.3Buckling of Ex. 2 wrap B Comp. Extruded piece 80 0.3 to 0.5 38.2 39.8Ex. 3 C Comp. Upset piece 80 0.3 to 0.5 38.4 39.7 Ex. 4

[0116] TABLE 4 Mechanical characteristics and machining test of forgedpart Fatigue Tensile characteristics characteristics 0.2% proof TensileFracture (room temperature) Observation of stress strength elongation10⁷ cycle machined Alloy Test (MPa) (MPa) (%) (MPa) surface Example BEx. 4 401 456 6.3 210 No tool scratch C Ex. 5 322 403 13.8 190 No toolscratch Comparative B Comp. 408 448 3.2 180 Tool scratch Example Ex. 3 CComp. 330 415 10.8 165 Tool scratch Ex. 4 H Comp. 410 458 3.8 170 Toolscratch Ex. 5 G Comp. 200 301 15.1 130 No tool scratch Ex. 6

[0117] According to the alloy material and the forging process of thepresent invention, mass-production of an aluminum alloy-made forgedscroll, in which formation of primary Si crystals which cause loweringof strength of the scroll and adversely affecting machining of thescroll, is suppressed can be achieved. According to the presentinvention, variation in wrap height of one forged scroll can be reducedand variation in mean wrap height between forged scrolls can also bereduced.

[0118] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A forged scroll part comprising an aluminum alloymaterial, comprising Al base material, Si in an amount of 8.0-12.5 mass%, Cu in an amount of 1.0-5.0 mass % and Mg in an amount of 0.2-1.3 mass%, wherein the scroll part substantially comprises no Si particleshaving a size of 15 μm or more, and a mean Si particle size is 3 μm orless.
 2. A forged scroll part comprising from an aluminum alloy materialcomprising Al base material, Si in an amount of 8.0-12.5 mass %, Cu inan amount of 1.0-5.0 mass % and Mg in an amount of 0.2-1.3 mass, and Niin an amount of 2.0 mass % or less; one or more species selected fromthe group consisting of Sr, Ca, Na, and Sb in a total amount of 0.5 mass% or less, or a mixture thereof; wherein the scroll part substantiallycomprises no Si particles having a size of 15 μm or more, and a mean Siparticle size is 3 μm or less.
 3. A forged scroll part produced from analuminum alloy material according to claim 1 or 2, wherein the scrollpart is subjected to solution heat treatment, quenching, and aging afterthe scroll part is subjected to forging.
 4. A process for producing analuminum alloy-made forged scroll part, which comprises a step forcasting an aluminum alloy material comprising Al base material, Si in anamount of 8.0-12.5 mass %, Cu in an amount of 1.0-5.0 mass % and Mg inan amount of 0.2-1.3 mass into a round bar having a diameter of 130 mmφor less through continuous casting; a step for cutting the aluminumalloy round bar into a stock material for forging; a step for subjectingthe stock material to upsetting at an upsetting ratio of 20-70% to forma work piece; a forging step for applying pressure onto the work piecewith a punch at a temperature of 300-450° C. to form a scroll wrap in adirection of punch pressing, wherein the forging step includes a singlestep in which a forged scroll part is press-formed while a back pressureis applied to the end of the scroll wrap part in a direction opposite tothat of a punch pressure.
 5. A process for producing an aluminumalloy-made forged scroll part, which comprises a step for casting analuminum alloy material Al base material, Si in an amount of 8.0-12.5mass %, Cu in an amount of 1.0-5.0 mass % and Mg in an amount of 0.2-1.3mass into a round bar having a diameter of 85 mmφ or less throughcontinuous casting; a step for cutting the aluminum alloy round bar intoa stock material for forging; a step for subjecting the stock materialto upsetting at an upsetting ratio of 20-70% to form a work piece; aforging step for applying pressure onto the work piece with a punch at atemperature of 300-450° C. to form a scroll wrap in a direction of punchpressing, wherein the forging step includes a single step in which aforged scroll part is press-formed while a back pressure is applied tothe end of the scroll wrap part in a direction opposite to that of apunch pressure.
 6. A process for producing an aluminum alloy-made forgedscroll part according to claim 4 or 5, wherein the aluminum alloymaterial comprises Al base material, Si in an amount of 8.0-12.5 mass %,Cu in an amount of 1.0-5.0 mass % and Mg in an amount of 0.2-1.3 mass,and Ni: 2.0 mass % or less; one or more species selected from the groupconsisting of Sr, Ca, Na, and Sb in a total amount of 0.5 mass % orless, or a mixture thereof.
 7. A process for producing an aluminumalloy-made forged scroll part according claim 4 or 5, further comprisingsubjecting the round bar to homogenization heat treatment at 480-520° C.for 30 minutes to four hours, to surface peeling or to homogenizationheat treatment and surface peeling after the casting.
 8. A process forproducing an aluminum alloy-made forged scroll part according to claim 4or 5, wherein work lubrication in which the work piece is coated withgraphite film in advance is carried out in combination with dielubrication in which a graphite-containing oily lubricant is applied toa die during forging.
 9. A process for producing an aluminum alloy-madeforged scroll part according to claim 8, further comprising heating thework piece at 100-500° C. and immersing the work piece into a lubricantsolution prepared by mixing and dispersing graphite powder into water tosubject the work piece to work lubrication with graphite film.
 10. Aprocess for producing an aluminum alloy-made forged scroll partaccording to claim 4 or 5, further comprising subjecting the scroll partto solution heat treatment, quenching, and aging after forging.
 11. Aprocess for producing an aluminum alloy-made forged scroll partaccording to claim 10, further comprising subjecting the surface of thescroll part to machining.